The combination of relatively high theoretical energy density, potentially long life, and low cost materials such as reported in the sodium-sulfur system high temperature batteries has been reported in the literature as suitable for low rate performance work such as electric road vehicle propulsion or load leveling of electric power supplies. The sodium-sulfur systems, first proposed in 1966, has had a great deal of effort expended in trying to develop a practical system. The basic operating principle involves the separation of two active molten materials, sodium and sulfur, by either a ceramic membrane of beta alumina or sodium glass, which at about 300.degree. C. or higher allows the passage of sodium ions that form with the sulfur any of the several polysulfides. The open circuit voltage of the system is at just over 2 volts, about the same as the lead-acid cell. Two formidable problems exist at the present time, viz., cracking of the separator and corrosion of the casing and seal.
Another somewhat similar system is the lithium-iron sulfide system, operating at about 450.degree. C. However, insufficient development has been done to date to demonstrate the widespread practicality of this system.
Another of the developments being pursued involves a lithium-based cell, in which the negative electrode is a lithium alloy (typically either lithium-aluminum or lithium-silicon), the positive electrode is an iron sulfide, and the electrolyte is a molten salt, such as the eutectic composition in the lithium chloride-potassium chloride system. Because of the high melting point of such salts, such cells must be operated in the temperature range of 400-500 degrees centigrade.
This requirement to operate at such high temperatures has several important disadvantages. One of these is that various degradation processes, such as corrosion of the cell container, seals, and other components are accelerated by such high temperatures. Another is that a substantial amount of energy is lost through heat transfer to the surroundings. Still another is that the voltage obtained from such cells is lower at elevated temperatures, due to the fundamental property of the negative temperature dependence of the free energy of the cell reaction. Furthermore, the higher the temperature of operation, the greater the potential problems related to damage to the cell during cooling to ambient temperature and reheating, whether deliberate or inadvertent. Differences in thermal expansion, as well as dimensional changes accompanying phase changes, such as the freezing of the molten salt, can cause severe mechanical distortions, and therefore damage to cell components.
Cells involving a lower temperature molten salt electrolyte have been investigated where the molten salt is based upon a solution of aluminum chloride and an alkali metal chloride. However, such salts are not stable in the presence of the respective alkali metals. As a result, an auxiliary solid electrolyte must be used to separate the alkali metal and the salt. One example of such a cell involves a molten sodium negative electrode, a solid electrolyte of sodium beta alumina, a molten aluminum chloride-sodium chloride salt, and either antimony chloride or an oxychloride dissolved in the chloride salt as the positive electrode reactant.
Such a cell can operate in the temperature range 150-250 degrees centigrade. It has the disadvantage of having to employ an electrolyte, which increases the cell impedance, as well as adding to the cost and complexity.
U.S. Pat. No. 4,751,161 to James et al discloses a non-aqueous lithium electrochemical cell utilizing lithium salts and an inorganic solvent mixture.
U.S. Pat. No. 3,844,837 to Bennion et al discloses a nonaqueous battery in which the anode may be lithium and/or graphite on which lithium metal is deposited and as a positive electrode a platinum cup filled with powdered K.sub.2 SO.sub.4 and graphite is utilized. The electrolytes disclosed are LiClO.sub.4, LiCF.sub.3 SO.sub.3 or LiBF.sub.4 dissolved in dimethyl sulfite.